In this specification, formation of a scribe line on a mother glass substrate of a liquid crystal display panel, belonging to glass substrates, which is a type of brittle material substrates, will be described.
An FPD such as a liquid crystal display panel or the like, which includes a pair glass substrates assembled together, is produced as follows. A pair of large sized mother glass substrates are assembled together, and then each mother glass substrate is scribed and broken into glass substrates included in the FPD. For scribing and breaking each mother glass substrate, a scribe line is formed in advance by a cutter on each glass substrate.
Recently, a method using a laser beam for forming a scribe line on a surface of a mother glass substrate has been put into practice. According to a method using a laser beam for forming a scribe line on a surface of a mother glass substrate, as shown in FIG. 6, a laser beam LB is directed from a laser oscillator 61 toward a mother glass substrate 50. The laser beam LB directed from the laser oscillator 61 forms an elliptical laser spot LS along a predetermined line for forming a scribe line (hereinafter, referred to as a “a scribe line formation line”) SL on the mother glass substrate 50. The mother glass substrate 50 and the laser beam LB directed from the laser oscillator 61 are moved with respect to each other along a longitudinal direction of the laser spot LS.
The mother glass substrate 50 is heated by the laser beam LB to a temperature which is lower than a softening temperature at which the mother glass substrate 50 is melted. Thus, a surface of the mother glass substrate 50 having the laser spot LS formed thereon is heated without being melted.
Toward an irradiation area irradiated with the laser beam LB and in the vicinity thereof on the surface of the mother glass substrate 50, a cooling medium such as, for example, cooling water can be sprayed from a cooling nozzle 62, so as to form a scribe line. On the surface of the mother glass substrate 50 irradiated with the laser beam LB, a compression stress is generated by the heating by the laser beam LB, and a tensile stress is generated by the cooling medium sprayed onto the surface. Thus, a tensile stress is generated in the vicinity of the area where the compression stress is generated. Therefore, a stress gradient is generated between the area having the compression stress and the area having the tensile stress, based on the respective stresses. On the mother glass substrate 50, a vertical crack is formed along the scribe line formation line SL from a notch TR which is formed in advance in an end area of the mother glass substrate 50.
FIG. 7 is a schematic projection showing an irradiation state of the laser beam LB on the mother glass substrate 50 which is scribed by a scribing apparatus. FIG. 8 is a plan view schematically showing a physical changing state of the mother glass substrate 50.
The laser beam LB oscillated from the laser oscillator 61 forms an elliptical laser spot LS on the surface of the mother glass substrate 50. The laser spot LS has an elliptical shape, for example, having a longer diameter b of 30.0 mm and a shorter diameter a of 1.0 mm. The laser beam LB is directed such that the longer axis thereof is along the scribe line formation line SL.
In this case, the laser spot LS formed on the mother glass substrate 50 has a higher thermal energy strength in an outer perimeter area thereof than that in a central portion thereof. Namely, each of the ends of the laser spot LS in the direction of the longer axis has the maximum energy strength. Such a distribution of thermal energy strength is obtained by converting a Gaussian distribution of thermal energy strength. Accordingly, the thermal energy strength is maximum at each of the ends in the direction of the longer axis which is located on the scribe line formation line SL. The thermal energy strength in the central portion of the laser spot LS interposed between the ends is lower than the thermal energy strength at each of the ends.
The mother glass substrate 50 can relatively move along the direction of the longer axis of the laser spot LS. Accordingly, the mother glass substrate 50 is first heated with a high thermal energy strength at one of the ends of the laser spot LS, next heated with a low thermal energy strength in the central portion of the laser spot LS, and then heated with a high thermal energy strength while moving along the scribe line formation line SL. After that, the cooling water from the cooling nozzle 62 is sprayed toward an area corresponding to an end of the laser spot LS, for example, a cooling point CP on the scribe line which is away from the end of the laser spot LS by a distance L of 0 to several millimeters.
Thus, a temperature gradient is generated between the laser spot LS and the cooling point CP. As a result, a large tensile stress is generated in an area opposite to the laser spot LS with the cooling point CP interposed therebetween. Utilizing this tensile strength, a vertical crack is generated in the mother glass substrate 50 in a thickness direction t from the notch TR formed at an end of the mother glass substrate 50 along the scribe line formation line.
The mother glass substrate 50 is heated by the elliptical laser spot LS. In this case, heat is conveyed in the vertical direction from the surface to the interior of the mother glass substrate 50, with a high thermal energy strength at one end of the laser spot LS. Since the laser spot LS moves relative to the mother glass substrate 50, a portion of the mother glass substrate 50 which is heated by a leading end of the laser spot LS is heated by the low thermal energy strength at the central portion of the laser spot LS and then again heated with a high thermal energy strength at a trailing end of the laser spot LS.
Thus, the surface of the mother glass substrate 50 is first heated with a high thermal energy strength, and while the surface of the mother glass substrate 50 is heated with a low thermal energy strength, the heat is conveyed to the interior thereof without fail. At this point, the surface of the mother glass substrate 50 is prevented from being continuously heated with a high thermal energy strength, which protects the surface of the mother glass substrate 50 from melting. After that, when the mother glass substrate 50 is heated again with a high thermal energy strength, the heat permeates into the interior of the mother glass substrate 50 without fail. Thus, a compression stress is generated on the surface and in the interior of the mother glass substrate 50. A tensile stress is generated by cooling water being sprayed toward the cooling point CP in the vicinity of the area in which the compression stress is generated.
When the compression stress is generated in the area heated by the laser spot LS and the tensile stress is generated at the cooling point CP by the cooling water, a large tensile stress is generated in an area opposite to the laser spot LS with the cooling point CP interposed therebetween. Such a tensile stress is generated by the compression stress generated in a thermal expansion area between the laser spot LS and the cooling point CP. Utilizing this tensile strength, a blind crack is generated from the notch TR formed at an end of the mother glass substrate 50 along the scribe line formation line.
When the blind crack acting as the scribe line is formed in the mother glass substrate 50, the mother glass substrate 50 is provided to the next breaking stage. In the breaking stage, a force is applied to both sides of the blind crack of the mother glass substrate 50 so as to generate a bending moment, which causes the blind crack to extend in the thickness direction of the mother glass substrate 50. Thus, the mother glass substrate 50 is scribed and broken along the blind crack formed along the scribe line formation line SL.
With such a scribing apparatus, it is necessary to increase the difference between the compression stress generated by the laser spot LS and the tensile stress at the cooling point CP, in order to form a vertical crack by a stress gradient between the heating by the laser spot LS formed on the surface of the mother glass substrate 50 and the cooling at the cooling point CP. In order to sufficiently perform the heating by the laser spot LS and the cooling by the cooling point CP, it is necessary to reduce the moving speed of the mother glass substrate relative to the laser spot LS and the cooling spot CP. As a result, a problem occurs in that the formation efficiency of the vertical crack is lowered.
In the case where an edge of the mother glass substrate 50, at which the heating by the laser spot LB along the scribe line formation line is started, is rapidly heated by an end of the laser spot LS, as shown in FIG. 9(a), there is an undesirable possibility that an uncontrollable crack CR is formed in the mother glass substrate 50 at a position ahead of the laser spot LS.
At the edge portion of the mother glass substrate 50, a stress remains when the mother glass substrate 50 is scribed and broken into a predetermined shape. The residual stress is released by the rapid heating by the laser spot LS, resulting in generation of the crack. The crack CR formed at a position ahead of the laser spot LS in this manner is uncontrollable and cannot be formed along the scribe line formation line.
Also in the case where an edge portion of the mother glass substrate 50, at which the heating by the laser spot LS is terminated after a blind crack BC is formed along the scribe line formation line, is rapidly heated by an end of the laser spot LS, as shown in FIG. 9(b), there is an undesirable possibility that an uncontrollable crack CR is formed from a side surface of the mother glass substrate 50 in a direction opposite to the moving direction of the laser spot LS. This crack CR is uncontrollable and cannot be formed along the scribe line formation line.
The present invention, made to solve these problems, has an object of providing a scribing method and a scribing apparatus for forming a scribe line on a brittle material substrate such as a mother glass substrate or the like, efficiently and without fail.
Another object of the present invention is to provide a scribing method and a scribing apparatus for scribing a brittle material substrate which can prevent formation of an uncontrollable crack at an edge portion of the brittle material substrate.